Strips on the screenGeneral

In my galaxy s4, several times I have seen strips with weird patterns in the lower half of the screen when I unlock it by swiping on the lock screen. It happens for a fraction of a second. My wife also told that the same thing happens in her phone too. It happens may 10% of the time, does not bother. But I am afraid. Is it a hardware defect? It happens during the time when my home screen appears after swiping the lock screen.